Ethylenically unsaturated compounds, such as acrylate derivatives, can be polymerized by exposure to actinic radiation, typically ultraviolet light, in the presence of a photoinitiating system. Photoinitiating systems include a photoinitiator, which is a compound capable of forming mono-radicals (molecular fragments having one unpaired electron) upon exposure to radiation. The mono-radicals can be formed via various mechanisms, such as .alpha.-cleavage or hydrogen abstraction, when the photoinitiator is exposed to radiation. An example of a conventional mono-racial formed by an .alpha.-cleavage mechanism is illustrated below: ##STR1##
The resultant mono-radicals can initiate polymerization by reacting with the desired compound to be polymerized to form active growing polymer chains.
Numerous mono-radical forming photoinitiators are commercially available. Despite their advantages, conventional mono-radical forming photoinitiators can suffer various disadvantages. For example, during free racial polymerization, the rate constant for termination (k.sub.t) can be several orders of magnitude larger than the rate constant for propagation (k.sub.p) ("Principles of Polymerization," G. Odian, McGraw-Hill, N.Y., 1970, page 171-2), particularly during the early stages of polymerization when the viscosity of the polymerization medium can be low. Premature termination can occur, for example, if the active radical polymer chain couples with another polymer chain or with an initiator radical. Premature termination can in turn adversely impact cure rates, molecular weight and molecular weight distribution of the resultant product, and the like.
Still further, cured articles formed using mono-radical photoinitiators can include residual fragment molecules. Residuals present in the cured article can create offensive odors and can degrade the physical properties of the article, such as light fastness, coloration, and resistance to oxidative degradation. In addition, residuals can be extracted or leach out of the cured article or migrate to the surface of the article.